L-arginine is the natural precursor of endothelium-derived nitric oxide, a mediator in the regulation of vascular tone by endothelial cells. The present investigation was undertaken to determine whether increased availability of nitric oxide precursor in the endothelial cells can potentiate the response to endothelium-dependent vasodilators. For this purpose, 12 normal subjects (age 49+7 years; 7 males and 5 females) were studied with infusion of acetylcholine, an endothelium-dependent vasodilator and sodium nitroprusside, a direct smooth muscle dilator, before and after the infusion of L-arginine. The effect of D-arginine (an isomer that is not a precursor of nitric oxide) on the response to acetylcholine was studied in 8 of these subjects. Drugs were infused into the brachial artery and the response of the forearm blood flow was measured by strain gauge plethysmography. At the doses infused (40 microm/min), L-arginine or D-arginine did not change blood flow. However, the vasodilator response to acetylcholine observed with L-arginine was significantly greater than that observed with acetylcholine alone. Conversely, L-arginine did not modify the response to sodium nitroprusside. Also, the response to acetylcholine was not modified by D- arginine. These findings indicate the availability of precursor of endothelium-derived nitric oxide is a rate-limiting step in endothelium- dependent vascular relaxation in humans.